That's interesting. I'd like to think I didn't crack the thermostat housing. I'll find out on Tuesday or Wednesday, once the mechanic has pulled it out.

My mechanic thinks I should have the car towed. I'm going to start a new thread to see if we can get more opinions before I have to move it on Monday.

 

VICTORY!

Now that I got the bolt unseized, the mechanic swapped in the new thermostat no problem. I had not cracked the thermostat housing. In fact, I don't know whether the thermostat or the housing were leaking or not; the drips onto the ground may have been residual from when I originally loosened the bolts. Who knows? But it doesn't matter. Problem solved in time for a 2 week car trip.

Thanks to everyone who posted, especially to hoonk, who has a lot of expertise and took a lot of time to help me.